268 KOOD, ON THE PRACTICAL APPLICATION OT 



prevents the proper stereoscopic union of the two photographs. 

 On this account I have generally adopted a different plan : — 

 the object is placed on an extra stage, Avhich can be inclined 

 Pjg 2 from 5° to 10% as seen in profile in the 



woodcut, Fig. 3. It is photographed first 

 at one angle, then at the other. In prac- 

 tice the manipulation is easy, and no par- 

 ticular difficulty is experienced from the fact that the extreme 

 right and left hand portions of the field are thrown slightly 

 out of focus. High and low powers can he used eqvially well. 

 The second negative should be taken immediately after the 

 first, before the illumination has altered. I do not know that 

 stereographs of microscopic objects have actually been taken 

 by other experimenters, though this may easily be the case. 



Living orc/anisms offer the photographer some difficulties, 

 by their constant motion about the field and in and out of the 

 focus. It becomes necessary to adopt a plan by which the 

 image can be thrown on the sensitive plate the very instant 

 after the animalcule has been brought into focus. The fol- 

 lowing method has been used by me with success, to obviate 

 this particular difficulty : — a plate of glass, with parallel sides, 

 is introduced at au angle of 45° into the tube outside of the 

 camera ; it reflects an image of the object to the ground glass 

 -p at G, rig. 3, which is placed so that an 



-p ' equally sharp image of the same object is 



formed at g'. The sensitive plate is in- 

 troduced at g', the flap at f being closed ; 

 jj , with one hand the operator, by the aid of 

 the image at g, focuses on the animalcule ; 

 just as this is efl^ected, the plate is exposed 



^ 



t 



3G. by the other hand turning the flap. If 



the collodion is sensitive, a second, or 

 less, suffices to give an image ; if a longer exposure be de- 

 sired, the image of the animalcule on the ground glass at g 

 can be watched, and the exposure prolonged till the creature 

 begins to change its position. The real difficulty, in the case 

 of living organisms, is found in the fact that all parts of them 

 do not lie in the same focus ; this, in fact, is one of the most 

 important difficulties connected Avith the whole subject of 

 microscopic photography. But the introduction of a slight 

 modification in the ordinary compressorium removes it in 

 many cases ; the plate of glass on which the objects rest, in- 

 stead of being plane, is made slightly convex, by the use of a 

 spectacle-lens of rather long focus. Objects to be examined 

 are placed near the point of contact, and pressure applied as 

 usual when they are brought nearly into the same plane. 



